Hitherto, nonaqueous electrolyte secondary batteries such as lithium secondary batteries have been widely used as power supplies of electronic devices and the like.
In recent years, the realization of high capacities for nonaqueous electrolyte secondary batteries has been desired. An example of a method for realizing a high capacity for a nonaqueous electrolyte secondary battery is to increase a charging voltage. For example, in the case where lithium cobalt oxide is used as a positive electrode active material of a nonaqueous electrolyte secondary battery, when the nonaqueous electrolyte secondary battery is charged up to 4.3 V on a metallic lithium basis, the capacity of the nonaqueous electrolyte secondary battery becomes about 160 mAh/g. On the other hand, when the nonaqueous electrolyte secondary battery is charged up to 4.5 V on a metallic lithium basis, the capacity of the nonaqueous electrolyte secondary battery becomes about 190 mAh/g.
An increase in the charging voltage of a nonaqueous electrolyte secondary battery may cause a problem in that a nonaqueous electrolyte is easily decomposed by the positive electrode active material. In particular, when a nonaqueous electrolyte secondary battery is charged at a high charging voltage at a high temperature, a nonaqueous electrolyte is more easily decomposed.
PTL 1 discloses a method for producing a positive electrode active material, the method including an adhesion step of allowing a phosphoric acid compound to adhere to compound oxide particles containing lithium and nickel, and a heating step of heat-treating the compound oxide particles to which the phosphoric acid compound is adhered. PTL 1 proposes that, for example, the charging current capacity of a secondary battery be increased by using the positive electrode active material produced by this method.